This invention relates to a method and an apparatus for fusion splicing of carbon-coated optical fibers and for providing reinforced performance of the fused splicing in an optical data transmission fiber.
Of the various optical fibers known today, silica based optical fibers are generally used with a coating of an organic material, such as synthetic resin. The purpose of the coating is to protect the exterior surface of the fibers from being damaged by encounters with foreign objects in the environment, such as dust particles and sharp objects, and thereby to incur degradation in fracture strength. However, this type of coating cannot prevent the diffusion of environmental substances, such as water and small molecules such as hydrogen, into the fiber. If water molecules are adsorbed onto the surface of silica based optical fiber under stress, a fatigue phenomenon can occur, leading to a time-dependent degradation in mechanical strength.
Further, if hydrogen diffuses into the silica based optical fiber, light absorption loss due to molecular vibration of hydrogen molecules will increase; as well, absorptive losses due to OH radicals, formed by the reaction between dopants P.sub.2 O.sub.5, GeO.sub.2 and B.sub.2 O.sub.3, can also increase.
To prevent such performance degradation, an optical fiber construction shown in FIG. 1 has been proposed. A silica based optical fiber 1, consisting of a core 1a and a cladding 1b, is coated with a carbon film coating 2 of 10-100 nm thickness, made of carbonaceous materials such as amorphous carbon, followed further with an outer organic layer of resin coating 3 to comprise a carbon coated optical fiber 4. The carbon film coating 2 on the optical fiber 1 is generally formed by means of chemical vapor deposition (CVD) process.
In the meantime, the demand for long-distance transmission of signals by optical fiber is increasing, and since such fibers can only be produced in a finite length, the fibers must be spliced to build a long-distance optical communication line. The fiber splicing techniques include connector and fusion methods, but generally, arc fusion method, which involves butting the fiber ends and fusing the silica based optical fiber by arc discharge, is accepted as being the most durable of the available techniques.
In practicing the fusion splicing technique, the resin coating 3 must be removed from the fiber 4 before abutting the ends of the fibers and performing fusion splicing. In the process of fusion splicing, the heat of fusing has an effect of oxidizing the carbon and destroying the coating 2 in the regions near the weld. Such a splicing region of fiber 1 is unprotected and becomes susceptible to absorption of moisture from the atmosphere in the splicing region. Such exposed splicing regions were the cause of serious concern, because adsorption of water vapor or exposure to water drops leads not only to long-term degradation of the mechanical strength of the splicing, but also to an increase in signal transmission losses.
Furthermore, the carbon coating layer is oxidized at the time of the fusion splicing, and there is a danger that the oxidization gas thereof will damage the optical fiber and thus lead to a decrease in strength.